of their respective yield strengths at the surface and 

 at depths of 2,500 and 6,000 feet for different 

 periods of time. 



The majority of the specimens were deformed by 

 bowing to obtain the desired tensile stress in the 

 central 2-inch length of the outer surface of the 

 specimen. Many of these specimens, butt-welded by 

 the TIG process, were positioned such that the trans- 

 verse weld bead was at the apex of the bow in the 

 2-inch length. Other specimens, 6 x 12-inch, had a 

 3-inch-diameter circular weld bead placed in the 

 center. The stresses induced by the welding operation 

 were not relieved in order to retain the maximum 

 residua] stresses in the specimens. Still other speci- 

 mens were in the shape of welded rings, 9-5/8 inches 

 outside diameter, which were deformed different 

 amounts in order to induce tensile stresses in the 

 periphery at the ends of the restraining rods. 



The results of the stress corrosion tests are given 

 in Table 83. There were no stress corrosion cracking 

 failures of any of the alloys, both unwelded and 

 butt-welded, stressed at values equivalent to as high as 

 75% of their respective yield strengths for 180 days 

 of exposure at the surface, 402 days at the 2,500-foot 

 depth, and 751 days at the 6,000-foot depth, except 

 for the butt-welded 13V-llCr-3Al alloy. The 

 unrelieved butt-welded 13V-llCr-3Al alloy failed by 

 stress corrosion cracking when stressed at values equi- 

 valent to 75% (94,500 psi) of its yield strength after 

 35, 77, and 105 days of exposure at the surface in the 

 Pacific Ocean. The stress corrosion cracks were in the 

 heat-affected zones at the edges of and parallel to the 

 weld beads. 



The butt-welded 6 x 12-inch specimens of 

 13-V-llCr-3Al alloy failed by stress corrosion during 

 398, 540, and 588 days of exposure at the surface 

 due to the unrelieved residual welding stresses. The 

 stress corrosion cracks were perpendicular to and 

 extended across the weld beads from side to side. 



The 6A1-4V alloy rings stressed as high as 60,000 

 psi (approximately 50% of its yield strength) did not 

 fail by stress corrosion cracking during 402 days of 

 exposure at the 2,500-foot depth. 



Alloys 75A, O.lSPd, 5Al-2.5Sn, 7Al-2Cb-lTa, 

 6Al-2Cb-lTa-lMo, 6A1-4V, and 13V-1103A1 were 

 exposed with an unrelieved 3-inch-diameter circular 

 weld bead in the center of 6 x 12-inch specimens. 

 Only the 13V-llCr-3Al alloy failed by stress corro- 

 sion cracking because of the residual welding stresses. 

 Failure by stress corrosion cracking occurred first 

 after 181 days of exposure at the surface. Thereafter, 

 failures first occurred during 189 days of exposure 

 when partially embedded in the bottom sediments 

 and during 751 days of exposure in the seawater at 

 the 6,000-foot depth. At the 2,500-foot depth the 

 first failure occurred during 402 days of exposure in 

 the seawater. The cracks in all cases extended radially 

 across the weld beads. In some cases, the cracks 

 changed direction by 90% and propagated circumfer- 

 entially around the outside of the weld bead. In 

 general, the 13V-llCr-3Al alloy was more susceptible 

 to stress corrosion cracking in seawater at the surface 

 than at depth in the Pacific Ocean. 



7.3. MECHANICAL PROPERTIES 



The effects of exposure in seawater on the 

 mechanical properties of the titanium alloys are given 

 in Table 84. The mechanical properties of the 

 titanium alloys were not adversely affected. 



226 



